The present invention relates generally to variable optical density lenses and in particular to liquid crystal lenses in which the optical density is varied as a function of ambient light intensity.
One type of liquid crystal spectacle lens with a rapid response to changes in ambient light is described in U.S. Pat. No. 4,279,474 and another is disclosed in our International patent application, PCT/GB 93/00119, in which we describe an electrical circuit for controlling the duty cycle of a waveform for driving a liquid crystal lens in dependence upon ambient light levels, in order to adjust the amount of light transmitted through the lens, the circuit including means for adjusting the response time of the circuit to changes in ambient light. That invention consists of a means for varying the rate at which the duty cycle and hence the optical density changes in response to changes in ambient light, and, in addition, comprises an arrangement for setting the threshold at which darkening starts and a means for obtaining a desired darkening characteristic above the threshold.
However, practical difficulties arise in implementing the method proposed in U.S. Pat. No. 4,279,474 for two interrelated reasons, namely user comfort and the unavailability of suitable liquid crystal cells. The operating principle outlined in U.S. Pat. No. 4,279,474 involves operating the liquid crystal cell in one of two states, a state of maximum transmission and a state of minimum transmission. The average value of the transmission depends on the proportions of time spent in the two states. It was argued in U.S. Pat. No. 4,279,474 that provided the switching frequency of the liquid crystal cell exceeded a certain value, known as the critical flicker fusion frequency, the wearer would perceive the average value of transmitted light. The critical flicker fusion frequency varies with light level according to the Ferry-Porter law and varies from 10 to 45 Hz over the range of light levels of interest in U.S. Pat. No. 4,279,474.
Human sensitivity to flicker has been studied in detail (see Lighting and Research Technology 6, (1974) 127, G W Brundrett). Brundrett reports that the critical flicker frequency also depends on the amplitude of the flicker compared to the mean light level. Sensitivity to flicker above the critical flicker fusion frequency was noted and led to headaches, dizziness and other side effects. The sensitivity of humans to flicker, based on electroretinogram data, has also been measured (see Abstracts of the Annual Meeting of the Association for Research in Vision and Opthalmology, 1988,Vol.29 D S Greenhouse, S M Berman, I L Bailey, and T Raasch). Although the study indicates that there was still a measurable retinal response at 200 Hz, it was less than one thousandth of the response at the critical flicker fusion frequency. Therefore, to avoid undesirable side-effects, when using the method employed in U.S. Pat. No. 4,279,474, the switching frequency should be at least 200 Hz.
To implement this approach with a switching frequency of 200 Hz, the switching time of the cell needs to be short compared to the times in the two transmission statesxe2x80x94a switching time of 20% of the minimum time in a state could be taken as a guide. The minimum time in a state will be the time in the state of minimum transmission when the liquid crystal cell is giving its highest average transmission, or the time in the state of maximum transmission when the liquid crystal cell is giving its lowest average transmission. A ratio of average values of transmission of 4:1 could be achieved by varying the proportions of time in the state of maximum and minimum transmissions from 1:4 to 4:1 and would give a minimum transmission similar to that found in conventional sunglasses of fixed transmission. At a switching frequency of 200 Hz that corresponds to a minimum time in a state of 1 ms. Therefore a cell switching time of less than 200 xcexcs is needed.
However, nematic cells of the type proposed in U.S. Pat No. 4,279,474 have minimum switching times of about 10 ms and could not therefore be used. An alternative is the use of ferroelectric cells, which exhibit switching times of 100 xcexcs or less but have practical disadvantages. Ferroelectric cells are, at present, not sufficiently robust for use in sunglasses and suffer irreversible damage if squeezed. The thickness of the liquid crystal is usually less than that in nematic cells, and needs to be more precisely controlled, making the manufacture of ferroelectric cells more difficult. In addition, a higher drive voltage is needed, up to 15V compared to the 5V used for nematic cells and to achieve effective operation a complex drive waveform is often employed. The generation of this kind of waveform is unattractive in solar powered eyewear, where it is desirable to use the minimum power at a low voltage to minimize the size of the solar cell array.
Accordingly, there is a need for another method of operating liquid crystal lenses, particularly of the nematic type, which alleviates the problems of flicker.
According to the present invention there is provided a liquid crystal lens device having an electrical circuit comprising means for detecting the ambient light level, and means for controllably varying the optical transmission of the liquid crystal lens dependent on said ambient light level, characterised by
means for controlling the duty cycle of the voltage driving the lens so as to cause the liquid crystal lens to operate in the transition region, between maximum and minimum optical densities.
Preferably, the RMS value of the pulse voltage is controlled so as to be arranged to remain between the threshold values of the liquid crystal lens.
Advantages of this method of operation are that a cell with slow response characteristics can be used and that flicker in the transmitted light at the driving frequency is very small. This thus avoids problems inherent in driving cells between states of maximum and minimum light transmission.
The transmission of a nematic liquid crystal cell depends on the root mean square (RMS) of the applied voltage over a wide range of voltage waveforms. In the case of a rectangular waveform (which is convenient to generate), the RMS and hence the transmission, will depend on the amplitude of the waveform and the square root of the duty cycle, at least for duty cycles of a few percent or greater.
The approach of varying the RMS by changing the amplitude, but maintaining an essentially constant duty cycle, has been exploited in sunglasses using LC cells as lenses (see H.Seki, Y.Masuda and Y.Itoh, Proc.Soc. for Information Display 32-3(1991) pp191-195). A serious drawback with this approach is that nematic LC cells show a rapid change in transmission over a small range of RMS voltage, demanding sensitive and stable control of the amplitude of the applied waveform, on which the RMS depends directly. In consequence, where the amplitude is determined by a supply voltage susceptible to change, as in solar powered spectacles, the resulting darkening characteristic is unlikely to be near the optimum. Although improvement would be possible by including circuitry, typically an analogue amplifier, to vary precisely the amplitude of the drive waveform, it is not practical to include analogue circuits in the type of low power consumption application specific integrated circuits (ASICs) that are desirable for solar powered sunglasses.
There are several advantages in the design and operation of the drive circuitry for LC lenses when the duty cycle of the drive waveform is varied. The RMS of the waveform is a function of the square root of the duty cycle, permitting a more gradual variation of RMS value and hence cell transmission. Avoiding the need to vary the amplitude of the drive waveform allows the design of a simpler drive circuit, using standard digital CMOS techniques. Such circuits can be readily incorporated into low power ASICs, which are advantageous in minimizing the power requirements of the system to enable the specification of a small and unobtrusive solar cell as the power source. To avoid unwanted changes in the RMS value of the drive waveform where the supply voltage varies, as it would in solar powered sunglasses, a simple voltage regulator can be employed. Alternatively, the amplitude could be allowed to change with supply voltage and the duty cycle compensated to maintain the RMS vale of the waveform constant.
The control of flicker in the transmitted light is important to avoid unpleasant visual effects for the wearer and in order to minimize the risk of headaches and other effects. The amplitude and frequency components of the waveform of the flicker in transmitted light depend on the time-response characteristic of the LC cell, the nature of the drive waveform and the local gradient of the transmission against RMS voltage characteristic. All other things being equal, the slower the response of the LC cell, the smaller the flicker in transmitted light. For a given LC cell, the most favourable condition with a rectangular drive waveform is a 50% duty cycle, but this will not be maintained in a system where the duty cycle is varied to alter the transmission of the LC cell. For a waveform of constant period, the flicker will grow as the time between the high to low and low to high transition increases with reducing duty cycle. An improvement is to divide the time for which the drive is high into several shorter pulses, whilst maintaining the same RMS value for the waveform. An alternative solution is to reduce the period of the drive waveform, i.e. increase its frequency, as the duty cycle falls. If the time for which the drive waveform is low does not change with duty cycle, the flicker will be reduced. A penalty with both techniques is that the number of transitions per second would increase, requiring more power to charge and discharge the capacitance of the LC cell.
It is presently proposed to employ conventional nematic cells with the circuit of the invention.